In recent years, a laser module (semiconductor laser module) having a laser diode (semiconductor laser) is variously developed in an optical communication field. For example, the laser diode is an optical component used in large quantities as a light source for a signal and a pumping light source for an optical amplifier. The laser module is an optical module in which a laser beam from the laser diode is optically coupled to an optical fiber.
In FIG. 19, one example of the laser module is shown by a YZ cross-section along its optical axis Z-direction. The laser module shown in FIG. 19 has a package 26, a thermo module 5 stored into this package 26, and a base 1 mounted to this thermo module 5.
A laser diode 3, a heat sink 24, lenses 2, 4, a photodiode 7 and a photodiode fixing portion 22 are mounted onto the base 1. Each of these elements mounted onto the base 1, and the base 1 are cooled elements cooled by the thermo module 5 as a temperature adjuster.
The thermo module 5 has a cooling side substrate 17 as a cooling portion, and a heating side substrate 18 arranged oppositely to this cooling side substrate 17. Plural peltier devices are spaced from each other between the cooling side substrate 17 and the heating side substrate 18.
A ferrule 49 held by a sleeve 48 is fixed to one end side of the package 26. A connecting end face side of the optical fiber 50 for optical transmission is inserted and fixed to this ferrule 49. Light outputted from one end 30 side of the laser diode 3 is incident to the optical fiber 50 for optical transmission through the lens 2, and is supplied in a predetermined desirable use through the optical fiber 50.
Light outputted from the other end 31 side of the laser diode 3 is incident to the photodiode 7 through the lens 4. The photodiode 7 functions as a monitor portion for monitoring the intensity of received light.
Since heat is generated from the laser diode 3 driven, the laser diode 3 is cooled by the thermo module 5. The above cooled elements on the thermo module 5, i.e., the base 1 and plural elements arranged on the base 1 are cooled simultaneously.
An unillustrated thermistor for LD is arranged in the vicinity of the laser diode 3. The cooling operation using the thermo module 5 is performed on the basis of a detecting temperature of the thermistor for LD. Namely, the cooling side substrate 17 of the thermo module 5 is cooled and the temperature of the laser diode 3 is held at a preset temperature by controlling an electric current flowed to the thermo module 5 such that the detecting temperature of the thermistor for LD is set to the preset temperature.
The laser diode 3 is an optical component having a temperature dependent property. Namely, an oscillating wavelength from the laser diode 3 depends on the temperature. Holding the temperature of the laser diode 3 at the preset temperature as mentioned above stabilizes the oscillating wavelength of the laser diode 3.
Recently, dense wavelength division multiplexing transmission has been vigorously considered in the optical communication field. The wavelength division multiplexing transmission is a transmission system for transmitting plural multiplexed optical signals through one optical fiber. A laser module stabilized for a long period with respect to the wavelength of the optical signal has been strongly required to apply the laser module to this wavelength multiplexing transmission system.
The laser module having a wavelength monitor for stabilizing the oscillating wavelength from the laser diode was proposed in Japanese Patent Laid-Open No. 2000-56185 to satisfy the above requirement. This laser module is constructed by arranging a wavelength filter, a photodiode for wavelength control, a peltier device, etc. as shown below.
Namely, in this proposed laser module, for example, as shown in FIG. 20, the laser diode 3 is arranged within the package 26, and a wavelength monitor portion 13 arranged on the other end 31 side of-this laser diode 3 monitors the oscillating wavelength of the laser diode 3. The wavelength monitor portion 13 has a beam splitter 35, an optical filter 6 for selectively transmitting the preset wavelength, and a photodiode 7 (7a, 7b). The optical filter 6 is formed by an Etalon filter, etc. The optical filter 6 is an optical component having the temperature dependent property. Accordingly, the wavelength monitor portion 13 is also an optical component having the temperature dependent property.
The laser diode 3 is arranged on the base 11. This base 11 and the base 1 of the wavelength monitor portion 13 are mounted onto the thermo module 5 for making the temperature adjustment of the wavelength monitor portion 13 and the temperature adjustment of the laser diode 3.
Similar to the laser module shown in FIG. 19, a ferrule 49 held by a sleeve 48 is also fixed to one end side of the package 26 in this optical module. The connecting end face side of an optical fiber 50 for optical transmission is inserted and fixed to this ferrule 49.
In the construction shown in FIG. 20, light outputted from the other end 31 of the laser diode 3 is changed to collimated light by a lens 4, and is incident to the wavelength monitor portion 13. One portion of the light incident to the wavelength monitor portion 13 is transmitted through a beam splitter 35 as a dividing portion for dividing the laser beam into two lights, and the remaining portions are reflected.
The light reflected on the beam splitter 35 is received by the photodiode 7 (7a), and the light intensity of the laser diode 3 is monitored. Further, the light transmitted through the beam splitter 35 is transmitted through the optical filter 6, and is received by the photodiode 7 (7b) for monitoring the wavelength.
In the optical module shown in FIG. 20, oscillating wavelength control of the laser diode 3 is performed on the basis of light receiving intensity of the photodiode 7 (7a, 7b).
Since heat is generated from the laser diode 3 driven shown in FIG. 20, an operation for cooling the laser diode 3 by the thermo module 5 is performed. The wavelength monitor portion 13 is also cooled by this cooling operation.
The optical transmission characteristics of the optical filter 6 and the oscillating wavelength from the laser diode 3 originally have the temperature dependent property. Therefore, the construction shown in FIG. 20 stabilizes the oscillating wavelength of the laser diode 3 by holding the temperatures of the wavelength monitor portion 13 and the laser diode 3 at the preset temperature as mentioned above.